Poor grades in school can have various causes. For one thing, students may have money problems. If they need to work long hours to make money, they will have less time to study. Another cause of poor grades may be the relationship problems. A student may be not happy with family problems or the lack of friends. That unhappiness can also harm schoolwork. A final cause of poor grades may be study problems. Some students have never learned how to take good notes in class, or how to study a textbook. Without such study skills,their grades are likely to suffer. What is the main idea of this paragraph?